Det är inte helt välkänt att sjuhundra tusen människor dog i Centraleuropa år 2003, bland annat som en följd av extrema värmeböljor. Enligt professor Harrison Fraker är detta bland annat en följd av att städer blir allt varmare och att allt för få åtgärder har riktats mot att arbeta med mikroklimat i arkitektur och stadsbyggnad. Enligt Fraker är färgsättning, vegetation, vatten, täthet, byggnadsprestanda och material sådana designelement som direkt påverkar temperaturen i staden och därför måste vara genomtänkta ur dessa perspektiv. En sådan inriktning leder inte bara till bättre komfort och hälsa utan visar också vägen för en ny ekonomisk potential.
Harrison Fraker är professor i arkitektur och urban design vid Berkley University of California, College of environmental design
Design attention on urban heat island effects (UHI)
The dramatic impacts of climate change can be seen in cities all over the globe. Devastating floods from severe storms and extreme heat waves and droughts are the new normal, caused by the increased energy (heat) in the global weather system. Much attention has been paid to the devastating impact of severe storms on cities and their infrastructure (see B.I.G.’s winning entry for protective infrastructure around Lower Manhattan , red kom: Aktuell plan i detta nummer). Nonetheless just as threatening to the health and well-being of cities, but not as well known, are the increased frequency of severe heat waves, and in general, the impact of increased urban heat island (UHI) effects. Not many people are aware that 700,000 people died in central Europe as a result of the extreme heat wave in the summer of 2003, exacerbated by the urban heat island effect. Recognizing this threat, Brian Stone Jr., in his book: The City in the Coming Climate, argues that, “known formally as the urban heat island effect, landscape change in cities (from urbanization) is emerging to be the principle climate-related threat to human health”. It is generally recognized that design solutions to address the dual threats of severe storms and heat waves is an interrelated problem; that it will involve both mitigation of the causes and adaptation to the impacts of climate change to make our cities more resilient and livable. It is also recognized that this is an integrated, whole-systems design problem that must engage all forms of urban infrastructure: energy, water, waste, waste water and all forms of transportation, and including the public space of cities land use planning and zoning. It is a “wicked problem” that challenges the very structure of how we govern, plan, design, build, operate and maintain our cities because it demands that urban agencies collaborate across strict legal boundaries. Yet, while it is a challenge, it presents a unique opportunity for urban design to formulate how the public realm can be reconceived and redesigned to be part of an integrated whole system solution. Increasingly research and design attention is focusing on how to mitigate and adapt to increases in the heat island effect and severe heat waves, especially in Asia and the global south. It is a complex local microclimate design challenge that is both region and site specific for every city in the world.
Design strategies and the urban section
For the purposes of urban design it is important to understand that the urban heat island (UHI) effect is the difference in temperature caused by urbanization between a city and its surrounding natural landscape and that it is caused by the following variables:
- Reduction in evaporative cooling of ground surfaces - impermeable surfaces
- Low surface reflectivity – dark surfaces
- Reabsorption of solar radiation reflected from building surfaces – urban canyons
- Waste heat from buildings, transportation and people
- Reduced natural cooling from local winds – dense urban fabric
The study of urban microclimates and design strategies to mitigate extremes has a long history going back to the designs of traditional climate responsive cities, employing – multiple forms of shading, material properties like thermal mass, methods of promoting natural ventilation and evaporative cooling. More recent research has quantified the impact of reintroducing dense vegetative cover. A study of Atlanta Georgia shows that the UHI will increase average high temperatures from 32-40 C and extreme temperatures could climb into the 50 C, threatening human health. On the other hand doubling tree cover will reduce these extreme temperatures by 7 C, back into a less threatening range. In addition a study by Lawrence Berkeley National Lab shows that tree planting (11 million) and reflective roofing (30% white roofs) will lower extreme temperature in Los Angeles by 3 C. Even a small intervention, a 5% increase in green roofs in Toronto is shown to lower extreme summer temperatures by 1 C. Shading and changing the reflectivity of cities are well established, cost effective strategies, yet their potential for transforming the form, experience and environmental performance of the public realm is underdeveloped in urban design. When the causes of the urban heat island (UHI) are examined, they call attention to the design of every surface in the city: the streets, sidewalks, parks, facades, roofs, courtyards and parking areas, etc., both public and private. While cities are typically studied and analyzed in plan, when environmental performance (see Figure 1) to reduce the UHI becomes a design driver, the urban section (see Figure 2) becomes even more important in imagining and exploring design options. To date, many innovative green design strategies have been implemented that contribute to the environmental performance of the public realm. In most cases their performance benefits both the building on which it is applied and it’s surrounding public space, acting as a reciprocal climate mediator between inside and outside. However, when all of the performance impacts are considered, these strategies have many more co-benefits that are of great value and have yet to be fully recognized or monetized.
Towards an integrated whole-system design
Most of these strategies have been applied in isolation, as separate projects. What if they were re-conceived at a larger scale, as a neighborhood or citywide integrated system? Their multiple performance characteristics and co-benefits would have the advantage of dramatically improving the microclimates of entire cities, both adapting and mitigating climate change, creating a much more livable city, while delivering economic and health co-benefits. Implementing such a neighborhood or citywide system that changes the environmental performance of public space is not easy. It would require a complicated, public-private collaboration; yet, several cities have begun the process. A green area ratio - a minimum vegetative cover requirement from a menu of green options, is being tried in Germany; and Seattle is implementing a green factor ordinance. Obviously, detailed designs would depend on the specific climate, urban fabric and economic capacity of each city or neighborhood. But, interestingly, the designs do not have to be limited to what can be required of buildings. All these strategies present a new tool kit of urban design options to reshape the public space and environmental performance of cities, but their systemic design potential needs to be realized. Just increasing the vegetative cover of cities is not a panacea for responding to climate change. The design and extent of any green strategies have to take into account multiple climate factors, especially the impact of local winds. In some climates the green infrastructure should be designed to block winter winds, in others, to promote the cooling effects of increased wind speeds in hot conditions. In many hot climates a serious challenge in mitigating the UHI effect is finding water to irrigate any new tempering, green infrastructure. In such cases, an integrated whole-system design approach can provide the answer by locally cleaning and recycling wastewater (sewage), using new technologies, to provide the necessary water for irrigation.
Creating new forms of economic development
Both the most promising and yet underdeveloped dimensions of these strategies are their economic potential. A study in Phoenix showed that changing the urban paving material, creating extensive tree canopy, adding green facades on the lower floors of buildings, and adding green or white roofs in a nine-block area would lower temperatures by 5 C. This change created an oasis in the city, promoting economic activity, increasing sales tax revenue and the savings in air conditioning costs from the buildings would pay for the improvements in 2 years. The feasibility and economic potential of these strategies is not limited to cities with developed economies. In fact the benefits may be greater in those cities with developing economies where the impact of climate change will be the most severe, especially in cities of the global south. These strategies have the potential to create local, internal economies. They could be instrumental in creating local urban agriculture production, building material production, energy fuels, and many other material resources, products and markets, generating new economic opportunity. The public, institutional, NGO and informal frameworks to enable these economies are just emerging. They are awaiting further inventions and need to be encouraged, facilitated and implemented.
There are many examples of design strategies that can be applied directly and independently to the public space of cities. They can involve creating a light-weight framework for vegetation or agriculture and the canopy of the public space could have seasonal shading fabrics deployed, much like decorative lighting in the Christmas season.
A new vocabulary of urban form
Urban design as a means to redesign and transform the environmental performance of urban space is in its infancy. Many of the strategies are proven, but as isolated projects. Their systemic performance is under researched, untried and their extraordinary design potential under explored. Unlocking the creative design potential imbedded in these strategies and finding innovative applications creates a whole new tool kit, a new vocabulary of urban form. It has the potential to give new identity and sense of place to our streets, neighborhoods and urban districts; one that not only adapts and mitigates climate change, but also enhances the experience and wellbeing in peoples’ everyday lives.